US6370397B1 - Search window delay tracking in code division multiple access communication systems - Google Patents

Search window delay tracking in code division multiple access communication systems Download PDF

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US6370397B1
US6370397B1 US09/070,778 US7077898A US6370397B1 US 6370397 B1 US6370397 B1 US 6370397B1 US 7077898 A US7077898 A US 7077898A US 6370397 B1 US6370397 B1 US 6370397B1
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Prior art keywords
delay
search window
error
mean
signal
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US09/070,778
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English (en)
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Branislav M. Popović
Frank Georg
Matthlas Schulist
Göran Klang
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Unwired Planet LLC
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Telefonaktiebolaget LM Ericsson AB
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Priority to US09/070,778 priority Critical patent/US6370397B1/en
Assigned to TELEFONAKTIEBOLAGET LM ERICSSON reassignment TELEFONAKTIEBOLAGET LM ERICSSON ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: GEORG, FRANK, SCHULIST, MATTHIAS, KLANG, GORAN, POPOVIC', BRANISLAV M.
Priority to TW088105529A priority patent/TW428375B/zh
Priority to CA002330926A priority patent/CA2330926C/en
Priority to CNB998081914A priority patent/CN1135725C/zh
Priority to AU43026/99A priority patent/AU755811B2/en
Priority to PCT/SE1999/000709 priority patent/WO1999057819A1/en
Priority to KR1020007012168A priority patent/KR100689993B1/ko
Priority to EP99948574A priority patent/EP1082820B1/en
Priority to JP2000547705A priority patent/JP4242563B2/ja
Priority to DE69932929T priority patent/DE69932929T2/de
Priority to ARP990102034A priority patent/AR016244A1/es
Priority to US09/320,638 priority patent/US6731622B1/en
Publication of US6370397B1 publication Critical patent/US6370397B1/en
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7073—Synchronisation aspects
    • H04B1/7085—Synchronisation aspects using a code tracking loop, e.g. a delay-locked loop
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7113—Determination of path profile
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
    • H04B1/7117—Selection, re-selection, allocation or re-allocation of paths to fingers, e.g. timing offset control of allocated fingers

Definitions

  • the present invention relates to code division multiple access (CDMA) communication in cellular radio telephone communication systems, and more particularly, to a multipath search processor employed in a CDMA RAKE type receiver.
  • CDMA code division multiple access
  • Direct sequence code division multiple access allows signals to overlap in both time and frequency so that CDMA signals from multiple users simultaneously operate in the same frequency band or spectrum.
  • a source information digital data stream to be transmitted is impressed upon a much higher rate data stream generated by a pseudo-random noise (PN) code generator.
  • PN pseudo-random noise
  • This combining of a higher bit rate code signal with a lower bit rate data information stream “spreads” the bandwidth of the information data stream.
  • each information data stream is allocated a unique PN or spreading code (or a PN code having a unique offset in time) to produce a signal that can be separately received at a receiving station.
  • a specifically PN coded information signal is isolated and demodulated by correlating the composite signal with the specific PN spreading code associated with that specific information signal. This inverse de-spreading operation compresses the received signal to permit recovery of the original data signal and at the same time suppresses interference from other users.
  • a receiver may also receive multiple, distinct propagation paths of the same signal transmitted from a single transmitter source.
  • One characteristic of such a multipath channel is an introduced time spread. For example, if an ideal pulse is transmitted over a multipath channel, the received corresponding signal appears as a stream of pulses, each pulse or path having a corresponding different time delay, as well as different amplitude and phase.
  • Such a complex received signal is usually called the channel impulse response (CIR).
  • Multipaths are created in a mobile radio channel by reflection of the signal from obstacles in the environment such as buildings, trees, cars, people, etc.
  • the mobile radio channel is dynamic in the sense it is time varying because of relative motion affecting structures that create the multipaths. For a signal transmitted over a time varying multipath channel, the received corresponding multiple paths vary in time, location, attenuation, and phase.
  • a CDMA receiver in accordance with the present invention employs a multipath search processor that searches for and identifies the strongest multipaths along with their corresponding time delays.
  • a RAKE demodulator captures most of the received signal energy by allocating a number of parallel demodulators (called RAKE “fingers”) to the strongest multipath components of the received multipath signal as determined by the multipath search processor.
  • the outputs of each of the RAKE fingers are diversity-combined after corresponding delay compensation to generate a “best” demodulated signal that considerably improves the quality and reliability of communications in a CDMA cellular radio communications system.
  • the multipath search processor (sometimes referred to herein as simply “searcher”) used in accordance with an example embodiment of the present invention identifies the channel impulse response of a complex received signal in order to extract the relative delays of various multipath components.
  • the searcher also tracks changing propagation conditions resulting from movement of the mobile station or some other object associated with one of the multipaths to adjust the extracted delays accordingly.
  • the channel impulse response of a received multipath signal is estimated within a certain range of path arrival times or path arrival delays called a search window. All signals detected within the search window form the delay profile, but only those signals originated from the transmitter belong to the channel impulse response. The remaining received signals in the delay profile are noise and interference.
  • the delay profile is called power delay profile (PDP).
  • the channel impulse response is estimated very frequently so that delay variations, of the radio channel can be tracked.
  • the position of the channel impulse response within the search window frequently changes because of movement of the mobile station or other object motion as well as from frequency mismatch of the PN sequence generators used at the transmitter for spreading and at the receiver for de-spreading.
  • the position of the search window must be adjusted to keep the channel impulse response in the middle of the search window.
  • the present invention provides a search window delay tracking procedure for use in a multipath search processor of a CDMA radio receiver.
  • a channel impulse response is estimated for a received signal containing plural paths, each path having a corresponding path delay.
  • a search window defines a delay profile that contains (1) the plural multipath components of the received signal forming the channel impulse response (CIR) and (2) noise and interference signals at delays where the transmitted multipath components do not exist.
  • CIR channel impulse response
  • a mean or average delay is calculated for the estimated channel impulse response, and an error is determined between the mean CIR delay and a desired delay position corresponding to the center of the CIR search window. An adjustment is made to reduce that error so that the center of the search window and the mean CIR delay are aligned.
  • the error may be processed either linearly (in one embodiment) or non-linearly (in another embodiment) to minimize the error and to reduce an influence of noise.
  • a non-linear filtering process includes calculating a delay spread from the mean CIR delay calculated for successive iterations, each iteration corresponding to a processing cycle of the window delay tracking procedure corresponding to each new input. A difference is determined between the successive delay spreads.
  • a search window adjustment signal is set equal to the error signal if the difference is less than or equal to a threshold. Alternatively, the adjustment signal is set to zero if the difference is greater than the threshold. Consequently, if the delay spread in the current iteration is significantly different from the delay spread in the previous iteration, the new error sample calculated in the current iteration is considered unreliable, and no adjustment is made.
  • the present invention is applied to a radio base station that includes plural sectors, each one of the sectors having one or more directive antennas receiving a signal from a mobile station that contains multiple paths. Each path has a corresponding delay.
  • a multipath search processor at the base station includes plural channel estimators, one corresponding to each of the plural sectors. Each channel estimator generates a delay profile within a search window containing the actual channel impulse response as well as noise and interference.
  • a path selector in the multipath search processor selects paths with strongest signals from the delay profiles generated by each channel estimator and outputs a selected channel impulse response made up of the corresponding delay and power for each selected path.
  • a window tracking unit maintains alignment of a center of the selected channel impulse response and a center of the search window.
  • a demodulator demodulates the selected paths and combines the demodulated paths into a combined received signal. The window tracking unit adjusts the search windows in all channel estimators to maintain the center alignment as well as adapts the delays for the selected paths according to any search window adjustment.
  • the window tracking unit in particular calculates a mean delay from the selected paths, determines an error between the mean delay and the center of the search window, and adjusts the position of each search window in order to reduce the error.
  • a window tracking unit controller calculates a delay spread for each new selected channel impulse response, determines a difference between the successive delay spreads, and sets an adjustment signal proportional to the error signal if the difference is less than or equal to a threshold, or sets the adjustment signal equal to zero if the difference is greater than the threshold.
  • FIG. 1 is a function block diagram of a cellular radio communications system in which the present invention may be employed
  • FIG. 2 is a drawing illustrating multipath propagation between a mobile station and a base station
  • FIG. 3 is a graph showing an example multipath channel impulse response delay profile useful in illustrating principles of the present invention
  • FIG. 4 illustrates an example embodiment of a CDMA receiver in which the present invention may be employed
  • FIG. 5 illustrates a format of an example information signal as transmitted processed and demodulated by the receiver in FIG. 4;
  • FIG. 6 is a function block diagram of the multipath search processor shown in FIG. 4 in which the present invention may be advantageously employed;
  • FIG. 7 is a function block diagram of a selector from the multipath search processor shown in FIG. 6;
  • FIG. 8 is a function block diagram of one of the channel estimators of the multipath search processor shown in FIG. 6;
  • FIG. 9 is a function block diagram of the de-spreader in the channel estimator shown in FIG. 8;
  • FIG. 10 is a function block diagram of a window tracking unit in accordance with one example embodiment of the present invention.
  • FIG. 11 is a function block diagram of a window tracking unit in accordance with another example embodiment of the present invention.
  • FIG. 12 is a flowchart illustrating search window delay tracking procedures in accordance with one example embodiment of the present invention.
  • FIG. 13 is a flowchart illustrating further search window delay tracking procedures in accordance with an example embodiment of the present invention.
  • a representative, connection-oriented external core network shown as cloud 12 may be for example the Public Switched Telephone Network (PSTN) and/or the Integrated Services Digital Network (ISDN).
  • a representative, connectionless-oriented, external core network shown as cloud 14 may be for example the Internet. Both core networks are coupled to one or more service nodes. For simplicity, only a single service node is shown as a mobile switching center (MSC) node 16 that provides switching services. The mobile switching center 16 is connected to a plurality of radio network controllers (RNCs) 18 .
  • RNCs radio network controllers
  • Each radio network controller 18 establishes and releases a particular channel between one or more base stations (BSs) 20 and mobile station (MS) 24 including the selection and allocation of spreading codes and diversity handovers.
  • the base station 20 handles the wideband CDMA radio interface to mobile station 24 and includes radio equipment such as transceivers, digital signal processors, and antennas required to serve each cell and cell sector in the network.
  • each base station may include multiple sectors 22 , and each sector preferably includes two diversity antennas.
  • FIG. 2 illustrates a simplified, dynamic multipath propagation model. While multipath propagation must be addressed by both mobile stations and base stations, for description purposes only, the multipath example illustrates a signal being transmitted from a mobile station 24 to a base station 20 .
  • the transmitted signal is received at the base station 20 the by the diversity antennas in plural sectors 22 with each received signal having multiple paths P 1 , P 2 , and P 3 .
  • Path 1 is the direct, first received, and often the strongest path.
  • Path 2 is reflected off a stationary object such as a building.
  • Path 3 is reflected off a moving object such as an automobile.
  • the mobile station 24 may be also be moving.
  • the basic problem then for the receiver in the base station 20 is to identify each of these paths P 1 -P 3 to determine their magnitude and relative delay so the three paths may be diversity-combined taking into account their respective delays.
  • FIG. 3 illustrates a graph employed to help explain the principles of the present invention.
  • the vertical axis of the graph is received signal power.
  • the horizontal axis is delay time intervals related to the rate at which the received signal is sampled.
  • the waveform is the estimated channel impulse response and includes four peaks having a magnitude that exceed a detection threshold. Only the three peaks corresponding to paths P 1 , P 2 , and P 3 are valid multipaths. The fourth peak is a false peak, but because it exceeds the threshold, it is also identified as the path. Path 1 corresponds to delay ⁇ 1 , path 2 corresponds to delay ⁇ 2 , and path 3 corresponds to delay ⁇ 3 .
  • the width of the horizontal axis corresponds to a search window.
  • the length of the search window is sufficient to fully encompass the channel impulse response (all of the significant multipaths of the received signal) plus an additional offset so that the window is somewhat wider than the portion of the channel impulse response containing valid multipaths.
  • the search window is defined by the number of delay values used as starting positions for correlating the received signal with the PN code in order to cover the maximum expected delay of the last-arrived, detected multipath component with respect to the first-arrived, detected multipath component.
  • the number of complex samples corresponding to the maximum expected multipath delay is 160, and therefore, N window equals 160 delay positions.
  • the center of the search window is aligned with a mean or average delay value of the channel impulse response.
  • the mean delay is determined by averaging the delays of each of the multipaths of the channel impulse response.
  • the difference or error ⁇ between the center of the search window N window /2 (at delay position 80 in FIG. 3) and the mean delay location (at a delay position slightly less than 80) is detected and minimized by adjusting the location of the search window (or by making some other adjustment).
  • Receiver 50 includes a RAKE demodulator 54 having a plurality of RAKE finger demodulators (not shown) which receive inputs from a PN sequence generator 58 (i.e., a PN de-spreading code sequence) and from a timing control unit 56 .
  • the timing control unit 56 generates synchronization (SYNC) signals provided to the RAKE demodulator 54 and to a multipath search processor 60 also connected to the RAKE demodulator 54 .
  • Signals from two diversity antennas 0 and 1 for each six base station sectors (0-5) are input to respective automatic gain control (AGC) circuits 52 .
  • AGC automatic gain control
  • Each AGC circuit is connected to both diversity antenna signals to reduce the long term dynamic range of the received signal, thereby reducing the required number of bits for signal representation but at the same time preserving the information content of the signal. Analog-to-digital conversion can be performed before or after AGC and therefore is not explicitly shown in the figure.
  • the multipath search processor 60 calculates delay profiles for each of the sectors using those output samples as described in further detail below.
  • the signal samples are also provided to the RAKE demodulator 54 for de-spreading and combining.
  • the combined output signal is generated using a number of antenna signals from different sectors selected by the multipath search processor according to the strongest multipaths received by all of the base station sectors.
  • pilot symbols or other known signals transmitted from the mobile station are used by the base station to estimate the channel impulse response.
  • the base station needs to derive synchronization signals necessary to extract periodically inserted pilot symbols from the received signal samples.
  • Such initial synchronization may be obtained after a random access procedure employed by mobile radios over a known access channel used to acquire a traffic channel from the base station.
  • the base station is synchronized to the first-arrived, detected multipath signal component originated from the mobile station. That initially received synchronization signal is used to extract pilot symbols subsequently transmitted on the traffic channel. Further adjustment of the synchronization signal is the task of window delay tracking unit in the searcher.
  • FIG. 5 shows an example data format in which information is transmitted from the mobile station.
  • Information symbols are formatted at the highest level as consecutive superframes provided to appropriate spreading circuitry in the mobile station transmitter.
  • the superframe information is spread using a PN code assigned by the base station to the mobile station and transmitted over the radio interface.
  • Each superframe (which may be for example 840 milliseconds), may include for example 64 consecutive radio frames where each radio frame may be 10 milliseconds.
  • each 10 millisecond radio frame may include 16 time slots, and each time slot includes pilot or known symbols used for synchronization and channel symbols containing unknown information symbols to be demodulated and communicated to the base station.
  • each automatic gain control unit corresponds to the signal transmitted by the mobile station.
  • Each of the automatic gain control circuits 52 is connected to base station selector 62 which selects blocks of signal samples from both antenna signals from each base station sector.
  • base station selector 62 selects blocks of signal samples from both antenna signals from each base station sector.
  • Each base station sector has a corresponding one of M channel estimators 64 , where M equals the number of base station sectors.
  • the selector 62 extracts blocks of signal samples to be searched for known symbols, e.g., pilot symbols, and provides those blocks to their corresponding channel estimator 64 .
  • the channel estimators 64 perform code matched filtering with coherent and non-coherent integration of the code matched filter responses. In coherent integration, complex correlation values obtained in a number of successive time slots for the same delay of the block of received signal samples are added together. In non-coherent integration, the powers of coherently-integrated correlation values are summed.
  • the corresponding channel estimator delivers an average power delay profile corresponding to the estimated channel impulse response to the path selection unit 66 .
  • the path selection unit 66 discriminates between signal and noise samples in the M delay profiles and then selects a number of strongest path signals to be demodulated in the RAKE demodulator 54 .
  • the corresponding path delays and powers of the N paths selected by the path selection unit 66 are provided to a window tracking unit 70 .
  • the number N of selected paths should be equal to the number of RAKE fingers, but N can also be smaller if there are not enough paths with powers above the detection threshold. These selected paths form a selected channel impulse response as defined above.
  • the main function of the window tracking unit 70 is to keep the multipath channel impulse response in the middle of the search window.
  • the search window position is corrected using a search window position correction signal from the window tracking unit 70 provided to the timing control block 56 . By adjusting the phase, i.e., the state of the PN generators which is applied to the channel estimator 64 , the search window is effectively adjusted.
  • a chip synchronization unit 68 determines whether an initial synchronization process is completed, and if so, sets a chip sync flag. The chip synchronization unit 68 detects that chip synchronization has been achieved if there is at least one selected path, with arbitrary power P k , that exceeds a detection threshold in the path selection unit 66 .
  • the selector 62 is comprised of a sector selector 72 and a pilot selector 74 as shown in FIG. 7 .
  • the sector selector 72 selects which of the base station sectors is to be searched. For each selected base station sector, both sector antennas are searched concurrently.
  • M 2 channel estimators 64
  • the sector selector time multiplexes outputs of the different sector antennas.
  • the pilot selector 74 consists of M pilot demultiplexers 76 providing selected outputs to respective M channel estimators 64 .
  • Each pilot demultiplexer 76 extracts and buffers L buffer complex samples. Assuming a chip oversampling rate of four (i.e., four samples per chip) in the receiver, the length of the buffer in each pilot demultiplexer is provided by the following equation:
  • L buffer N pilot symbol * spreading factor * oversampling factor+ N window ⁇ 1 (1)
  • N pilot symbol is the number of known pilot symbols in each time slot
  • the spreading factor corresponds to the number of chips per symbol.
  • N window equals 160 complex samples (delay time intervals) which corresponds to a maximum expected delay of the multipath channel impulse response. Because the exact channel impulse response position is uncertain, an additional N window ⁇ 1 samples are stored along with the number of samples corresponding to the known pilot symbols.
  • each of the M parallel channel estimators 64 is shown in FIG. 8 .
  • the demultiplexed and buffered signal from corresponding pilot demultiplexer buffer 76 is correlated in a de-spreader 80 with the known, complex PN 1 /PN Q de-spreading code (the pilot sequence) to generate a complex correlation vector thereafter multiplied by a scaling factor in multiplier 82 which depends on the automatic gain control amount.
  • FIG. 9 shows in more detail the de-spreader 80 .
  • the down-sampled signal is multiplied chip-by-chip in the complex multiplier 94 with a segment of the complex conjugated and re-modulated spreading sequence provided form PN buffer and pilot re-modulator 93 .
  • the complex multiplication results are integrated in parallel I/Q integrators 95 over L chips to produce a complex correlation value.
  • the re-modulation of the spreading sequence is performed in order to remove the influence of the pilot symbol pattern (i.e., its information content) on the correlation value.
  • the same complex PN sequence (i.e., including both real PN 1 and imaginary PN Q sequences), is correlated using complex despreader 80 with successive sample offsets of the received signal stored in a corresponding pilot demultiplexer 76 buffer.
  • the complex correlation vectors from successive time slots are coherently integrated, (i.e., corresponding complex correlation values from successive time slots are added in phase), in a coherent accumulator 84 .
  • the absolute value of the output of the coherent integrator 84 is obtained and squared (block 86 ) to obtain a delay power spectrum (DPS).
  • DPS delay power spectrum
  • the delay power spectra are non-coherently integrated in a non-coherent accumulator 88 to produce a power delay profile (PDP) for each radio frame. Additional averaging of power delay profiles in sliding averager 90 is preferably performed to obtain an average PDP over a number of frames, having reduced noise/interference peaks.
  • two antenna diversity is implemented in each base station sector.
  • the average power delay profiles from the two antennas in the same sector are added and stored with a corresponding delay value.
  • the power values of the paths in the average PDPs are compared to a detection threshold, such as the horizontal dashed line shown in FIG. 3, and only those path power values that exceed the threshold are identified as valid paths.
  • the valid paths from all search sectors are compared, and the N strongest paths among them are sorted and selected according to descending order of their powers.
  • the delays ⁇ 1 ′, . . . , ⁇ N ′ and powers P 1 , . . . , P N of the selected paths are produced as the input signals for the window tracking unit 70 .
  • S N indicates which base station sectors and diversity antenna signals have been selected. This selection information is used in the RAKE receiver to select appropriate input signals. In addition, the average interference power for each sector is provided as an input signal for signal-to-interference ratio (SIR) estimation used for power control. The N strongest paths are then assigned to corresponding demodulation fingers in the RAKE demodulator for demodulation and diversity combination.
  • SIR signal-to-interference ratio
  • the window tracking unit 70 receives delays ⁇ 1 ′, . . . , ⁇ N ′ and powers P 1 , . . . , P N for the selected paths from the path selection unit 66 .
  • the window tracking unit 70 includes an error detector 100 which comprises a channel impulse response (CIR) mean position calculator 102 connected to a summer 104 which also receives as an input at a subtraction terminal the center location of the search window N window /2 measured in delay intervals.
  • the output of the summer 104 corresponds to an error signal ⁇ (m) in a processing cycle m being analyzed by controller 108 .
  • CIR channel impulse response
  • the mean delay value output by channel impulse response mean position calculator 102 is provided as an input to a channel impulse response delay spread calculator 106 along with the corresponding delays ⁇ 1 ′, . . . , ⁇ N ′ and powers P 1 , . . . , P N of the selected paths.
  • Examples of the search window, mean delay, delay spread, paths P 1 -P 3 , and delays ⁇ 1 - ⁇ 3 are labeled in FIG. 3 .
  • Controller 108 processes the error ⁇ (m) along with delay spread provided by the CIR delay spread calculator 106 to generate an adjustment signal A(m) provided both to a summer 110 and to an integrator 112 .
  • the adjustment signal A(m) is used to adjust the selected path delays ⁇ 1 ′, . . . , ⁇ N ′ which are provided to the RAKE demodulator so that appropriate delays can be applied to the RAKE finger outputs for coherent addition to generate the combined output signal.
  • Integrator 112 includes a summer 114 and a delay 116 for accumulating the outputs of the controller 108 and outputting a search window position correction signal W(m) provided to the timing and control unit 56 .
  • the timing control unit 56 controls the phase of the PN sequence generator 58 so that it is delayed or advanced according to the value of W(m).
  • the controller may simply be a linear low-pass filter and functions as a control loop filter.
  • a preferred choice of the loop filter bandwidth compromises the output noise variance and the loop tracking speed.
  • a second example embodiment of the present invention implements controller 108 using a non-linear, low-pass, error-filtering process referred to as “error verification” and will be described further below.
  • a third example embodiment of the window tracking unit 70 used in conjunction with the error verification embodiment is shown in FIG. 11.
  • a low-pass filter 120 filters the output of controller 108 before it is provided to summer 110 and integrator 112 .
  • the low-pass filter 120 smoothes the control signal output from the controller 108 , reducing the remaining noise in the control signal.
  • Adjust Search Window routine (block 130 ) shown in flowchart format in FIG. 12 .
  • Path delays and powers corresponding to those strongest paths selected for RAKE demodulation are processed to determine a channel impulse response (CIR) mean position or delay (block 132 ).
  • CIR channel impulse response
  • the CIR mean position is compared to the center of a search window to determine an error (block 134 ). That error is used to adjust the search window position relative to the CIR mean position as well as to correct each path delay used by the RAKE demodulator (block 136 ).
  • a delay spread is determined for the delays of the selected paths based on a difference between each selected path's delay and the mean delay (block 142 ). The delay spreads are then compared for successive iterations (block 144 ). A decision is made in block 146 if the difference between successive delay spreads is greater than or less than a threshold To. If so, the adjustment signal is set to zero (block 148 ). If not, a decision is made in block 146 whether the current delay spread equals zero. If not, the adjustment signal is set to the error (block 158 ).
  • the adjustment signal corresponding to the error is used to adjust path delays provided to the RAKE demodulator (block 150 ). If the current delay spread equals zero, the absolute adjustment signal value is compared with an average adjacent signal, and if the difference is larger than T 1 (block 159 ), the adjustment signal is set to a limit value T 1 (block 160 ). The adjustment signal is integrated (block 152 ) to generate a window control signal. The search window location is then adjusted in accordance with the window control signal (block 154 ).
  • ⁇ k ′ ⁇ (0, 1, . . . , N window ⁇ 1 ) are path delays
  • N is the number of selected paths
  • P k are the corresponding signal powers.
  • Each path delay is expressed as an integer number of positions (sampling time intervals) from the search window start position. The total number of searched positions within the search window is equal to N window .
  • the controller 108 processes the error signal ⁇ (m) in order to reduce the influence of noise, i.e., to prevent or reduce the movement of the search window due to an erroneous adjustment signal produced as a result of noise or interference.
  • the new error sample calculated in the current iteration is considered unreliable. This situation may happen when false paths caused by noise (interference) are detected, or when one or more valid paths temporarily disappears under the influence of Rayleigh fading.
  • condition C 0 (m) is given by
  • T 0 is a delay spread difference threshold.
  • the control signal A(m) will be zero only in a single iteration when the large change of delay spread is first detected. Afterwards the delay spread difference decreases to smaller values that permit adjustment of the search window.
  • the adjustment signal is produced as follows:
  • is an average of the adjustment magnitudes, which is calculated periodically within some chosen time interval, e.g.,
  • PN generator 58 which determines the relative phase of a PN sequence generator 58 with respect to the input signal via timing control unit 56 , i.e., a positive or negative time shift of the generated PN sequence. If the PN generator 58 is implemented using a shift-register with a feedback loop and a controllable clock frequency, the integrator 112 is not used and the adjustment signal A(m) is fed back to the PN generator 58 which itself functions as an integrator. In any case, a positive value of the control signal causes an additional delay of the generated PN sequence, and a negative value produces an advancement of the generated PN sequence. For practical implementation, it may be desirable to quantize the tracking control signal, so that it corresponds to an integer number of PN chips.
  • the window tracking unit 70 adapts the selected path delays ⁇ 1 ′, . . . , ⁇ N ′ according to the adjustments of the search window. Namely, there is a delay of one frame period before the searcher 60 determines the new path delays corresponding to the changed window position. During that time, the phase of the de-spreading PN sequence generated by the PN generator 58 will have already been adjusted, so for correct path combining in the RAKE demodulator 54 , it is necessary to temporarily adjust the already found path delays, until the newly-determined, correct path delays arrive from the path selection unit.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
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US09/070,778 US6370397B1 (en) 1998-05-01 1998-05-01 Search window delay tracking in code division multiple access communication systems
TW088105529A TW428375B (en) 1998-05-01 1999-04-07 Search window delay tracking in code division multiple access communication systems
JP2000547705A JP4242563B2 (ja) 1998-05-01 1999-04-29 符号分割多重アクセス通信システムにおけるサーチウィンドウの遅延トラッキング
CNB998081914A CN1135725C (zh) 1998-05-01 1999-04-29 在码分多址通信系统中用于搜索窗延迟跟踪的方法和设备
AU43026/99A AU755811B2 (en) 1998-05-01 1999-04-29 Search window delay tracking in code division multiple access communication systems
PCT/SE1999/000709 WO1999057819A1 (en) 1998-05-01 1999-04-29 Search window delay tracking in code division multiple access communication systems
KR1020007012168A KR100689993B1 (ko) 1998-05-01 1999-04-29 부호 분할 다중 접속 통신 시스템에서 검색 윈도우 지연추적 방법 및 장치
EP99948574A EP1082820B1 (en) 1998-05-01 1999-04-29 Search window delay tracking in code division multiple access communication systems
CA002330926A CA2330926C (en) 1998-05-01 1999-04-29 Search window delay tracking in code division multiple access communication systems
DE69932929T DE69932929T2 (de) 1998-05-01 1999-04-29 Verzögerungsnachführung mit Suchfenster in einem Kodemultiplexvielfachzugriffsübertragungssystem
ARP990102034A AR016244A1 (es) 1998-05-01 1999-04-30 Un metodo para determinar una senal de ajuste de seguimiento de demora de ventana de busqueda para un receptor radio celular de una disposicion decomunicacion de acceso multiple por division de codigo. una unidad de seguimiento de ventana de busqueda para llevar a la practica dicho metodo y una radi
US09/320,638 US6731622B1 (en) 1998-05-01 1999-05-26 Multipath propagation delay determining means using periodically inserted pilot symbols

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